瞬間 and 一瞬: an event's instant and a brief duration
Compare opening a door or photographing takeoff with a light briefly going out or a bird being briefly visible, separating event timing from duration.
An instant of opening or takeoff
扉を開けた瞬間、冷たい風が吹き込んだ。 reports cold wind blowing in the instant the door was opened. 瞬間, read しゅんかん, connects the door-opening event with the wind's entry.
鳥が飛び立つ瞬間を写真に収めた。 reports photographing the moment a bird took flight. The photographed target is that moment, not the bird's complete route or flight duration.
A brief outage and a brief sighting
電気が一瞬消えたが、すぐに元へ戻った。 reports the light going out for an instant and soon coming back on. 一瞬, read いっしゅん, describes the brief duration, while すぐに元へ戻った supplies its actual return.
窓の外に白い鳥が一瞬だけ見えた。 reports a white bird appearing outside the window for only a moment. 一瞬だけ limits its visibility; it does not limit the bird's existence to that interval.
Timing and short duration
The 瞬間 examples identify particular instants of opening and takeoff. The 一瞬 examples describe brief periods of a light being out or a bird being seen. Locate the associated event or state.
Do not assign an exact one-second duration to 一瞬. The photograph's timing, the light's return, and the brief sighting are stated separately, without a shared numerical duration.
How does photographing the bird's 瞬間 differ from seeing it for 一瞬?
Show the answer and why
The first identifies the event's moment photographed. The second describes a very brief sighting. Neither supplies an exact duration or the whole flight.